Alternative water sources will be needed to ensure there is enough in the coming decades and water will have to be used more efficiently, a report for the Dunedin City Council suggests.
It also said critical water pipelines were vulnerable to asset failure and both the Tahuna wastewater treatment plant and Musselburgh wastewater pumping station were highly exposed to failure.
Growth in parts of the city was constrained by capacity and poor performance of wastewater networks during wet weather.
The council is developing a strategic programme with a 50-year planning horizon. Councillors are due to discuss it at an infrastructure services committee meeting on Monday.
Challenges include the impact of climate change, ageing infrastructure, leakage of water into wastewater networks, regulatory requirements and increasing costs.
An increasing risk of drought, floods and damage to infrastructure was identified.
Existing networks and systems were assessed and this demonstrated Dunedin’s reliance on surface water sources.
Almost 80% of the city’s water came from the Deep Creek and Deep Stream supplies.
Many surface water sources were "over-allocated", the report said.
"All of Dunedin’s existing water sources will need new consents in the early 2040s and it is unlikely that we will be able to take as much water from existing sources as we currently do.
"More water will need to be left in streams and rivers for the health of the environment and water taken must be used more efficiently."
Groundwater was likely to be looked at as a water source, as well as a need to increase raw water storage.
The council said its wastewater networks performed adequately in dry weather.
Ageing pipes allowed groundwater and surface water to leak into wastewater networks during rain.
The main interceptor sewer was a critical asset at the end of its "theoretical useful life".
"Condition assessment of this asset is ongoing and, when completed, will confirm remaining life and help plan for its replacement," the report said.
Areas constrained for growth by the capacity and performance of wastewater networks included Kaikorai Valley, North East Valley, parts of the central city and Mosgiel.
The Musselburgh pumping station and pipelines from there to the Tahuna plant carried 64% of Dunedin’s wastewater — both facilities were in need of upgrades or replacement.
All wastewater consents would expire in the next eight years, the report said.
The Tahuna and Green Island wastewater treatment plants would have their consents expire in 2032.
"It is expected that higher standards of treatment will need to be met."
Stormwater infrastructure in South Dunedin was considered the most critical for the city, but there were also areas of high risk in Mosgiel, Kaikorai Valley, North East Valley, Sawyers Bay and Abbotsford.
More information about the programme is expected to be presented before the end of the year.
